1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Description of the Related Art
In recent years, resistive memory devices utilizing a variable resistor as a memory element are receiving attention as candidates to succeed flash memory. The resistive memory devices herein include resistive RAM (ReRAM), in a narrow sense, that uses a transition metal oxide as a recording layer and stores its resistance states in a non-volatile manner, as well as Phase Change RAM (PCRAM) that uses chalcogenide or the like as a recording layer to utilize the resistance information of crystalline states (conductors) and amorphous states (insulators).
Two kinds of operation modes in memory cells of resistive memory devices are known. In one kind, known as a bipolar type, a high-resistance state and a low-resistance state are set by switching a polarity of an applied voltage. In the other kind, known as a unipolar type, setting of the high-resistance state and the low-resistance state are made possible by controlling a voltage value and a voltage application time, without switching the polarity of the applied voltage.
The unipolar type is preferable for realizing a high-density memory cell array. This is because, in the case of the unipolar type, the cell array can be configured by overlapping a variable resistor and a rectifier such as a diode at crossing-points of bit lines and word lines, without using a transistor. Furthermore, arranging such memory cell arrays three-dimensionally in stacks enables a large capacity to be realized without causing an increase in cell array area (refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-541613).
In the case of unipolar type ReRAM, write of data to a memory cell is performed by applying for a short time to the variable resistor a certain voltage. As a result, the variable resistor changes from the high-resistance state to the low-resistance state. Hereinafter, this operation to change the variable resistor from the high-resistance state to the low-resistance state is called a setting operation. In contrast, erase of data in a memory cell is performed by applying for a long time to the variable resistor in the low-resistance state subsequent to the setting operation a certain voltage lower than that applied during the setting operation. As a result, the variable resistor changes from the low-resistance state to the high-resistance state. Hereinafter, this operation to change the variable resistor from the low-resistance state to the high-resistance state is called a resetting operation. The memory cell, for example, has the high-resistance state as a stable state (reset state), and, in the case of binary data storage, data write is performed by the setting operation which changes the reset state to the low-resistance state.
In such a semiconductor memory device, there is a need to accurately apply a voltage required in the setting operation and resetting operation of the memory cell.